Cut the vanilla pod in half, scrape out the pulp. Mix vanilla pulp, sour cream, eggs, egg yolks, vanilla sugar, sugar and custard powder. Take the puff pastry out of the fridge. Peel and quarter apples, remove core. Cut the quarter diagonally on the convex side
Unroll the puff pastry, remove from baking paper and fold into a rectangle (21 x 24 cm). On a floured work surface, roll out into a square (approx. 32 x 32 cm). Using the dough roller, place the puff pastry in a greased tart tin (28 cm Ø). Pour in the sand casting and place the apples with the cut side facing upwards. Bake in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: see manufacturer) for approx. 50 minutes, remove from oven and let cool down